| Title | Temperature and Compositional Changes in the Hydrothermal Fluids in the Mt. Amiata Geothermal Area: Evidence from Fluid Inclusion Data |
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| Authors | Giovanni Ruggieri, Chiara Giolito, Giovanni Gianelli, Maria Luce Frezzotti |
| Year | 2005 |
| Conference | World Geothermal Congress |
| Keywords | Hydrothermal minerals, fluid inclusions, Mt. Amiat geothermal field, Italy |
| Abstract | Two geothermal fields, Bagnore and Piancastagnaio, are in the southern sector of the Quaternary Amiata volcano (southern Tuscany, Italy). Fluid inclusions trapped in quartz, adularia and calcite found in core-sample in deep drilled wells (between 1690 and 3220 m below the ground level) of the geothermal fields provided information on the physical-chemical characteristics of circulating fluids in past. Three types of fluid inclusions related to the geothermal activity were recognized: type I (liquid-rich) inclusions trapped an aqueous fluid with CO2 concentration (1.27-2.68 mol/kg) that is significantly higher than that of present-day fluids (0.11-0.05 mol/kg). Type II (liquid-rich) inclusions formed after type I, and trapped a fluid with lesser CO2 (<0.85 mol/kg). Type III (vapor-rich) inclusions record an early fluid circulation under boiling conditions and in some samples coexist with type I inclusions. The decrease of CO2 concentration from type I inclusions to type II inclusions, and then to present-day concentrations can be related to boiling with gas loss and/or mixing. In general type I and II inclusions show variable salinities (0.11-3.90 mol/kg of Na+ and Cl-) and Th (150-335?C), which can also be associated with boiling and mixing processes. In particular, the data suggest that mixing between a hot low-salinity fluid and a cooler saline fluid may has occurred in samples where present-day temperatures are in the range 195-280?C. On the other hand, the present-day hottest areas (295-320?C) appear to be nearly in thermal stability from the time of fluid inclusion trapping and mixing processes are not evident. |